A typical prior art counter for particles suspended in a fluid is disclosed in U.S. Pat. No. 4,746,216 to K. Gross, assignor to Pacific Scientific Company. This patent describes a particle counter in which gas flows through a space within an enclosure, known as a view volume, monitored by a laser beam. The beam intersects the gas flow and illuminates particles flowing through the view volume. Light obscured by the particles, or scattered by the particles, is detected by an electronic detector sensitive to the obscuration or scattering of light. The resultant electrical signals are interpreted as particle counts.
The need for improved particle counters is largely driven by the semiconductor manufacturing industry. Silicon wafers have become larger in size while, at the same time, line widths and features on chips laid out on the wafer have become smaller. Chip size has become larger with more complex functions on each chip. Defects and particles which previously caused little harm now can render a large portion of a wafer useless. Thus, the role of small particles becomes increasingly important in monitoring quality of chip production. Air quality in clean rooms is especially critical at many stages of chip production. Semi-conductor companies routinely monitor air quality at all stages of chip production.
While various improvements have been made to particle counters themselves, including the optics, nozzle characteristics, lasers and laser cavities, detector attributes and signal electronics, little attention has been paid to the pumps which are used to maintain the air flow through the air through the view volume. Typically, pumps used in prior art particle counters use pistons, diaphragms or vanes to move a fluid carrier medium, such as air. Such pumps move relatively large volumes of the carrier medium at low cost. However, most prior art pumps are noisy, generate heat, and are somewhat inefficient. An object of the invention was to provide an improved pump for a particle counter which provides for quiet operation, relatively cool running and is efficient at flow rates and pressures used in particle counters.